Prepared in cooperation with the U.S. Department of State, the Mekong River Commission, 

Phnom Penh Autonomous Port, and the Cambodian Ministry of Water Resources and 

Meteorology

Hydrographic Survey of Chaktomuk, the Confluence of the 

Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, 

Cambodia, 2012

Scientific Investigations Report 2014–5227

U.S. Department of the Interior

U.S. Geological Survey
1

2

3

45

Cover photographs.

1. West bank of the Tonlé Sap River looking upstream, 

Phnom Penh, Cambodia, Februrary 8, 2011. Photograph 

by Benjamin J. Dietsch, U.S. Geological Survey (USGS). 

2. Survey of a bench mark at Chroy Changvar, Phnom 

Penh, Cambodia, April 24, 2012. Photograph by 

Brenda K. Densmore, USGS.

3. View of the Mekong River near Phnom Penh, 

Cambodia, from the upper deck of the hydrographic 

survey boat, April 20, 2012. Photograph by Benjamin J. 

Dietsch, USGS.

4. Bank of the Mekong River near Kday Takoy Temple, 

Phnom Penh, Cambodia, April 21, 2012. Photograph by 

Benjamin J. Dietsch, USGS.

5. Survey of a bench mark at Kday Takoy Temple, Phnom 

Penh, Cambodia, March 28, 2012. Photograph by Richard 

C. Wilson, USGS.
Hydrographic Survey of Chaktomuk, the 

Confluence of the Mekong, Tonlé Sap, 

and Bassac Rivers near Phnom Penh, 

Cambodia, 2012

By Benjamin J. Dietsch, Brenda K. Densmore, and Richard C. Wilson 

Prepared in cooperation with the U.S. Department of State, the Mekong River 

Commission, Phnom Penh Autonomous Port, and the Cambodian Ministry of 

Water Resources and Meteorology

Scientific Investigations Report 2014–5227

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Dietsch, B.J., Densmore, B.K., and Wilson, R.C., 2014, Hydrographic survey of Chaktomuk, the confluence of the 

Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012: U.S. Geological Survey Scientific 

Investigations Report 2014–5227, 23 p., http://dx.doi.org/10.3133/sir20145227.

ISSN 2328-0328 (online)
iii

Acknowledgments

Acknowledgment is extended to the many people and organizations that helped throughout the 

study. Special acknowledgment is given to Felix Seebacher, Erland Jensen, Hourt Khieu, Katry 

Phung, Paradis Someth, and Sameng Preap of the Mekong River Commission; Chantha Say and 

Bunny Pech of the Phnom Penh Autonomous Port; Toch Bon Vongsar and Horn Sovann of the 

Cambodian Ministry of the Water Resources and Meteorology; Mr. Sen and Mr. Sokhea, boat 

pilots; Ben Roohi, Terry Murphree, Timothy Mean, and Patrick Tran of the U.S. Embassy Phnom 

Penh, Cambodia; and Mark Dedomenic and Kofi Safo of the U.S. Department of State Diplomatic 

Pouch and Mail Facility. The Phnom Penh Autonomous Port helped greatly with logistics by 

providing local secure storage, skilled riverboat pilots, security guards, a skilled worker in metal 

fabrication, and a total-station operator. The authors also thank James Stefanov of the U.S. Geological Survey for his guidance, direction, and continued support.
iv

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Description of Study Area ...................................................................................................................2

Mekong River Basin ....................................................................................................................2

Hydrographic Survey of Chaktomuk.........................................................................................5

Methods of Investigation..............................................................................................................................6

Description of Equipment ....................................................................................................................6

Hydrographic Survey Methods ..........................................................................................................9

Field Procedures..........................................................................................................................9

Data-Processing Procedures ....................................................................................................9

Quality Assurance and Quality Control ...........................................................................................11

Hydrographic Survey Maps .......................................................................................................................12

Mekong River.......................................................................................................................................13

Bassac River........................................................................................................................................15

Tonlé Sap River....................................................................................................................................15

Summary........................................................................................................................................................15

References Cited..........................................................................................................................................20

Glossary.........................................................................................................................................................23
v

Figures

1. Map showing the Mekong River Basin and study area.........................................................3

2. Graph showing average, maximum, and minimum mean daily discharge 

of Mekong River at Mekong River Commission streamflow-gaging 

station 019802 at Kompong Cham, Cambodia, 1960–2002 ......................................................4

3. Map showing the confluence of the Mekong, Tonlé Sap, and Bassac Rivers...................5

4. Photographs showing a multibeam-echosounder mapping system ...................................8

5. Index map showing the hydrographic survey of Chaktomuk, the confluence 

of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 

April 21–May 2, 2012...................................................................................................................13

6. Map showing hydrographic survey of the Mekong River upstream from the 

confluence of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, 

Cambodia, April 30–May 2, 2012...............................................................................................14

7. Map showing hydrographic survey of the Mekong River at the confluence 

of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 

April 24–April 25, 2012 ................................................................................................................16

8. Map showing hydrographic survey of the Mekong River downstream from the 

confluence of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, 

Cambodia, April 21–April 23 and April 27–April 28, 2012 ......................................................17

9. Map showing hydrographic survey of the Tonlé Sap River upstream from the 

confluence of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, 

Cambodia, April 26, 2012............................................................................................................18

10. Graphs showing frequency distributions of the areas of riverbed elevations 

surveyed at Chaktomuk, the confluence of the Mekong, Tonlé Sap, and 

Bassac Rivers near Phnom Penh, Cambodia, April 21–May 2, 2012..................................20

Tables

1. Annual exceedance probability of occurrence of water levels at Chaktomuk..................6

2. Specifications of multibeam-echosounder mapping system equipment............................7

3. Location and location checks of bench marks used for hydrographic surveys of 

Chaktomuk near Phnom Penh, Cambodia, 2012 ....................................................................10

4. Summary by section of study area of total propagated uncertainty for the 

hydrographic survey of Chaktomuk near Phnom Penh, Cambodia, 

April 21–May 2, 2012...................................................................................................................12

5. Summary of riverbed elevations surveyed at Chaktomuk, Cambodia, 

April 21–May 2, 2012, and water-surface elevations at two streamflowgaging stations............................................................................................................................19
vi

Conversion Factors and Datum

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2

) 0.0002471 acre 

square kilometer (km2

) 247.1 acre

square meter (m2

) 10.76 square foot (ft2) 

square kilometer (km2

) 0.3861 square mile (mi2)

Volume

cubic meter (m3

) 264.2 gallon (gal) 

cubic meter (m3

) 0.0002642 million gallons (Mgal) 

cubic meter (m3

) 35.31 cubic foot (ft3)

cubic meter (m3

) 1.308 cubic yard (yd3) 

cubic meter (m3

) 0.0008107 acre-foot (acre-ft) 

cubic kilometer (km3

) 0.2399 cubic mile (mi3) 

Mass

ton 1.102 ton, short (2,000 pounds)

Flow rate

cubic meter per second (m3

/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per second (m3

/s) 35.31 cubic foot per second (ft3/s)

cubic meter per second (m3

/s) 22.83 million gallons per day (Mgal/d) 

Ratio

decibel (dB) 0.1 bel

Frequency

hertz (Hz ) 1 cycle per second

kilohertz (kHz) 1,000 cycles per second

Equivalent concentration terms

gram per liter (g/L) 1 part per thousand

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84).

Vertical coordinate information is referenced to Ha Tien 1960.

Elevation, as used in this report, refers to distance above the vertical datum.
Hydrographic Survey of Chaktomuk, the Confluence of the 

Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, 

Cambodia, 2012

By Benjamin J. Dietsch, Brenda K. Densmore, and Richard C. Wilson

Abstract

The U.S. Geological Survey, in cooperation with the 

U.S. Department of State, Mekong River Commission, Phnom 

Penh Autonomous Port, and the Cambodian Ministry of Water 

Resources and Meteorology, completed a hydrographic survey 

of Chaktomuk, which is the confluence of the Mekong, Tonlé 

Sap (also spelled Tônlé Sab), and Bassac Rivers near Phnom 

Penh, Cambodia. The hydrographic survey used a highresolution multibeam echosounder mapping system to map the 

riverbed during April 21–May 2, 2012.

The multibeam echosounder mapping system was 

made up of several components: A RESON SeabatTM 7125 

multibeam echosounder, an inertial measurement unit and 

navigation unit, data collection computers, and a Real-Time 

Kinematic (RTK) Global Navigation Satellite System (GNSS) 

base station. The survey area was divided into six survey subreaches and each subreach was surveyed within 3 days along 

survey lines oriented parallel to the flow direction. Complete 

coverage of the riverbed was the operational objective; however, to obtain broad spatial coverage, gaps between parallel swaths were permitted, especially in wide, shallow areas 

where multibeam swath widths were narrow. 

The survey was referenced to two existing bench marks 

with known geographic coordinates by establishing a GNSS 

base station on the bench marks each day and using real-time 

corrections from the base station to correct boat navigation 

data. The World Geodetic System of 1984 (WGS 84) ellipsoid was used during data collection to reference height, and 

data were adjusted to the local datum, Ha Tien 1960, during 

postprocessing.

The quality of hydrographic surveys was described by an 

uncertainty estimate called total propagated uncertainty (TPU). 

Calculations of TPU were completed for the hydrographic survey data resulting in the maximum TPU of 0.33 meters. The 

mean and median TPUs were 0.18 meters, and 99.9 percent of 

TPU values were less than 0.25 meters.

Detailed hydrographic maps of Mekong, Tonlé Sap, 

and Bassac Rivers showing the riverbed elevations surveyed 

April 21–May 2, 2012, referenced to Ha Tien 1960 were 

produced. The surveyed area included a 2-km stretch of the 

Mekong River between the confluence with the Tonlé Sap and 

Bassac Rivers, and extended 4 km upstream and 3.6 km downstream from the 2,000-m confluence stretch of the Mekong 

River. In addition, 0.7 km of the Bassac River downstream 

and 3.5 km of the Tonlé Sap River (from the confluence to 

Chroy Changvar Bridge) upstream from their confluence 

with the Mekong River were surveyed. Riverbed features 

(such as dunes, shoals, and the effects of sediment mining, 

which were observed during data collection) are visible on the 

hydrographic maps. All surveys were completed at low annual 

water levels as referenced to nearby Mekong River Commission streamflow-gaging stations. Riverbed elevations surveyed 

ranged from 24.08 m below to 1.54 m above Ha Tien 1960. 

Introduction

The Lower Mekong Basin (fig. 1) is home to more than 

60,000,000 people with 11,600,000 people living in Cambodia (Mekong River Commission, 2011). Because of the 

basin’s importance to this region, the Lower Mekong Initiative 

(LMI) was created as a result of a July 2009 meeting between 

the U.S. Secretary of State and the Foreign Ministers of the 

Kingdom of Cambodia, Lao People’s Democratic Republic, 

Socialist Republic of Vietnam, and the Kingdom of Thailand. 

Myanmar (Burma) formally joined the LMI in July 2012 

(Turnipseed, 2011). The People’s Republic of China is not a 

formal member of the LMI, but is a stakeholder in the initiative. The purpose of the LMI is to create integrated regional 

cooperation among the five Lower Mekong countries. The 

LMI serves as a platform to address complex, transnational 

development and policy challenges in the Lower Mekong 

Basin region. The member countries agreed to enhance cooperation and build upon their common interests in the areas of 

environment and water, health, education, and infrastructure 

development (U.S. Department of State, 2014). 

The LMI Environment and Water Pillar focuses on the 

development of a regional approach to sustainable environmental management and capacity development to manage 
2 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

shared natural and water resources. One element of the LMI 

environmental and water program is Forecast Mekong. Forecast Mekong is a project led by the U.S. Geological Survey 

(USGS) that provides data integration; predictive environmental, natural, and water resource models; and scientific 

visualization tools to help the LMI member countries make 

resource management decisions (Stefanov, 2012). Forecast 

Mekong provides planning tools to visualize and predict the 

consequences of water-resources development and river management. One Forecast Mekong project is the hydrographic 

survey of Chaktomuk, the confluence of the Mekong, Tonlé 

Sap (also spelled Tônlé Sab), and Bassac Rivers near Phnom 

Penh, Cambodia (fig. 1). The USGS, in cooperation with the 

U.S. Department of State, Mekong River Commission (MRC), 

Phnom Penh Autonomous Port, and the Cambodian Ministry 

of Water Resources and Meteorology, surveyed this area to 

collect data that could improve navigation and shipping safety 

and provide baseline scientific data to document natural and 

anthropogenic disturbances and geomorphic change in the 

confluence area. The hydrographic survey was completed in 

2012 and supports the goals of Forecast Mekong by providing 

data to help visualize the confluence and aid the development 

of planning tools. 

Purpose and Scope

The purpose of this report is to describe the methods and 

results of the 2012 hydrographic survey at Chaktomuk, the 

confluence of the Mekong, Tonlé Sap, and Bassac Rivers near 

Phnom Penh, Cambodia. The report includes background on 

the Mekong River Basin and a discussion of the objectives of 

the Forecast Mekong Project and the LMI.

Description of Study Area

The focus of the Forecast Mekong project includes the 

entire Lower Mekong River Basin (Lower Basin in fig. 1); 

however, the scope of the hydrographic survey is limited to 

the Chaktomuk area near Phnom Penh in southeast Cambodia (fig. 1). The Chaktomuk area was selected for this study 

because of the hydrologic importance of the location and its 

proximity to Phnom Penh, the capital city of Cambodia. The 

confluence of the Mekong, Tonlé Sap, and Bassac Rivers is 

known in the Khmer language as Chaktomuk, or “the four 

faces”, signifying the four branches of the three rivers. The 

French named the confluence the Quatre Bras, or “the four 

arms.” For most of the year, the Tonlé Sap River flows into the 

Chaktomuk, and the Bassac River flows out of the Chaktomuk 

to the South China Sea. During the wet season, the Tonlé Sap 

River flows away from the Chaktomuk and carries water from 

the Mekong River to the Tonlé Sap Lake, or the Great Lake, 

which is located northwest of Phnom Penh (fig. 1). The Tonlé 

Sap Lake serves as a flood- storage reservoir for the Mekong 

River and supports important wetlands, fisheries, and ricegrowing areas (Campbell and others, 2006). Sedimentation 

or other alterations at Chaktomuk could alter this unique and 

important hydrologic system. 

The confluence of the Mekong, Tonlé Sap and Bassac 

Rivers is ecologically and economically important to Cambodia. It serves as the primary source of drinking water for the 

approximately 1,500,000 citizens of Phnom Penh (U.S. Central Intelligence Agency, 2014), as well as a water supply 

for many industrial and commercial uses. Chaktomuk is the 

receiving water body for treated and untreated wastewater 

effluent. Chaktomuk serves as a transportation hub for the 

region. Vessels ranging from 5,000 deadweight tons sea-going 

cargo ships, to high-speed passenger boats, to small riverfishing boats can dock and transfer goods, materials, people, 

and food products at Phnom Penh Autonomous Port, which is 

an international port under the supervision of the Cambodian 

Ministry of Public Works and Transport and the Ministry of 

Economy and Finance. To facilitate shipping, Phnom Penh 

Autonomous Port maintains a continuous program of dredging 

Chaktomuk (Phnom Penh Autonomous Port, 2014). Typically, 

dredge spoils from the confluence are deposited near the head 

of the Bassac River creating reclamation land that has important economic value for commerce and industry within Phnom 

Penh (Chaktomuk Project Management Unit and DHI Water 

and Environment, 2002). 

Mekong River Basin

The Mekong River begins in the Tibetan Plateau of China 

at an elevation of approximately 4,500 meters (m) above mean 

sea level (Mekong River Commission, 2005). The river flows 

southeast for approximately 4,800 kilometers (km) (Mekong 

River Commission, 2005) through the countries of China, 

Myanmar (Burma), Laos, Thailand, Cambodia, and Vietnam, 

where it discharges into the South China Sea. The river basin 

area is approximately 795,000 square kilometers (km2), the 

mean annual discharge is approximately 14,500 cubic meters 

per second (m3

/s), and the annual runoff is approximately 

457 cubic kilometers (Mekong River Commission, 2005). 

Based on mean annual discharge at its mouth, the Mekong 

River is the tenth largest river in the world (Mekong River 

Commission, 2005).

The Mekong River typifies large tropical rivers driven 

by the monsoonal cycle, with large mean annual discharge in 

conjunction with well-defined monsoonal wet and dry seasons 

(Adamson and others, 2009). The wet season typically lasts 

from May until early October, with the greatest precipitation 

usually falling in August and September (Mekong River Commission, 2005). The magnitude of discharges during the wet 

season can be 20 times larger than discharges during the dry 

season (fig. 2). 

The Mekong Basin can be divided into two parts—the 

Upper Basin and Lower Basin (fig. 1). The Upper Mekong 

Basin, primarily located within the Chinese provinces (not 

shown) of Yunnan and Tibet, composes approximately 

24 percent of the total Mekong Basin. In the Upper Basin, 

the Mekong River is called the Lancang Jiang, and generally 
Introduction 3

Mekong Basin

Lancang Jiang (Mekong) River

Lower Basin

Upper Basin

Kratie

Pakse

Mekong River at 

Kompong Cham

(019802)

Tonlé Sap River 

at Prek Kdam

(020102)

0 meters

3,495 meters

Upper Basin

Lower Basin

Elevation, in meters

Hanoi

Bangkok

Vientiane

Phnom 

Penh

C H I N A

T H A I L A N D

C A M B O D I A

V I E T N A M

M Y A N M A R

( B U R M A )

LAOS

0 150 300 KILOMETERS

G U L F

O F

T O N K I N

A N D A M A N

S E A

I N D I A N

O C E A N

I N D I A

Tonlé Sap

Great Lake

Mekong River

Mekong River

S O U T H

C HI N A

S E A

0 150 300 MILES

Tonlé Sap 

River

Mekong

Delta

HIMALAYA RANGE

Map area

Tonlé Sap 

River

Mekong River Phnom Penh

Tonlé Sap

Great Lake

Bassac 

River

Chaktomuk

Tibetan 

Plateau

EXPLANATION

Major river

Other river

Capital city

City

Base from Global Land Cover Facility: Shuttle Radar Topography Mission, 2004

Horizontal coordinate information referenced to the World Geodetic System 

1984 (WGS 84) geographic projection

Vertical coordinate information referenced to the Earth Gravity Model, 1996

Nay Pyi Taw

Rangoon

Figure 1. The Mekong River Basin and study 

area (modified from Turnipseed, 2011).
4 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

flows through narrow and deep canyons (Mekong River Commission, 2005). The Upper Basin has a monsoonal climate that 

varies with topography and elevation. The seasonal distribution of precipitation follows the monsoon cycle, but overall 

amounts generally decrease towards the north in Tibet (not 

shown). Snowmelt runoff at the higher elevations is a large 

source of water during the dry season, particularly in April 

through May. The Upper Basin contributes approximately 

16 percent of the mean annual discharge, but more than 

24 percent of the discharge during the dry season originates 

from snowmelt (Mekong River Commission, 2014a). 

The Lower Mekong Basin approximately begins downstream from Yunnan Province (not shown), China. The 

Lower Basin widens in Thailand and Laos (fig. 1). The steep 

left-bank tropical tributaries (oriented looking downstream) 

in Laos contribute most of the flow of the Mekong during 

the wet season, whereas the low-relief right-bank tributaries in Thailand drain areas that generally receive less rainfall 

and contribute only 6 percent of the mean annual discharge 

(Mekong River Commission, 2005). At Kratie, in central 

Cambodia (fig. 1), the Mekong’s mean discharge is more than 

90 percent of the mean annual discharge at its mouth (Mekong 

River Commission, 2014a). In Cambodia, the Lower Basin is 

characterized by a wide and flat flood plain and contains the 

Tonlé Sap River and the Tonlé Sap Lake (also called the Great 

Lake). The Mekong, Tonlé Sap River, and Tonlé Sap Lake 

form a unique hydrologic system in which the Tonlé Sap River 

reverses flow twice a year depending on high or low stage of 

the Mekong River. When stage is low in the Mekong River, 

the Tonlé Sap River flows to the Mekong River. Conversely, 

when stage is high in the Mekong River, the Tonlé Sap River 

flows to Tonlé Sap Lake. Tonlé Sap Lake serves as a floodstorage reservoir for the Mekong River. The Tonlé Sap Basin 

has a drainage area of approximately 84,000 km2 and Tonlé 

Sap Lake is the largest fresh- water lake in Southeast Asia 

(Kummu and Sarkkula, 2008). Tonlé Sap Lake serves as an 

important agricultural and fisheries production area for Cambodia. At Chaktomuk, the Tonlé Sap River joins the Mekong 

River, and then splits into two main distributary channels consisting of the Mekong and Bassac Rivers (fig. 1). Chaktomuk 

also marks the beginning of the Mekong Delta that extends 

from Cambodia through Vietnam into the South China Sea 

(fig. 1) (Mekong River Commission, 2005). 

Figure 2. Average, maximum, and minimum mean daily discharge of Mekong River at Mekong River Commission streamflowgaging station 019802 at Kompong Cham, Cambodia, 1960–2002 (Mekong River Commission, 2014b).

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

Jan. Feb. Mar. Apr. May June July

Month

Aug. Sept. Oct. Nov. Dec.

Mean daily discharge, in cubic meters per second

Dry season Wet season Dry season

Maximum

Minimum

Average
Introduction 5

Hydrographic Survey of Chaktomuk

The Tonlé Sap and Mekong Rivers flow into the confluence from the northwest and north respectively, whereas the 

Mekong and Bassac Rivers flow out of the confluence to the 

southeast and south respectively (figs. 1 and 3). The confluence is located approximately 330 km upstream from the 

mouth of the Mekong River at the South China Sea, 3,870 km 

downstream from the headwaters of the Mekong River, and 

140 km downstream from Tonlé Sap Lake, and it is the origin 

point of the Bassac River. 

The climate conditions at Phnom Penh are determined by 

the monsoonal wet and dry cycle typical for the lowland plains 

of Cambodia. The wet season typically begins in May and 

ends in October. The dry season typically begins in November 

and ends in April with the least monthly average precipitation 

occurring in January. The average yearly rainfall at Phnom 

Penh is 1,470 millimeters (mm) (Mekong River Commission, 2014b) with the greatest average rainfall occurring in 

September. The hottest month is April, the coolest month is 

January, and the range of monthly mean temperatures is only 

3.5 degrees Celsius (°C) (Mekong River Commission, 2014b). 

The mean annual temperature is 27.7 °C (Mekong River Commission, 2014b).

The Mekong River at Kompong Cham (streamflowgaging station number 019802; Mekong River Commission, 

2014c) is approximately 77 km upstream from Phnom Penh, 

making it the closest discharge measurement station to Chaktomuk (fig. 1). The mean daily hydrograph at Kompong Cham 

has a consistent seasonal pattern (fig. 2) that is similar to other 

Lower Mekong River streamflow-gaging stations at Kratie, 

Pakse, and Vientiane (fig. 1). The May–June increase in 

streamflow is representative of the monsoon-driven transition 

from dry to wet seasons. 

The Mekong River at Kompong Cham streamflow-gaging 

station was started in 1960. The monthly mean discharge 

ranged from a minimum of 2,625 m3

/s in April to a maximum 

of 40,388 m3

/s in September. From 1960 to 2002, the lowest discharge on record was 1,947 m3

/s measured April 21, 

1993, and the peak of record was 69,025 m3

/s measured on 

August 17, 1978 (Mekong River Commission, 2014b). Typically, the maximum water levels occur in August to October 

Figure 3. The confluence of the Mekong, Tonlé Sap, and Bassac Rivers.

490,000 495,000

EASTING

500,000 505,000

1,275,000 1,280,000

NORTHING

1,285,000

Base from Esri Imagery Layer Package, 2012

Horizontal coordinate information referenced to the 

Universal Transverse Mercator projection, zone 48 North 0 1 2 MILES

Mekong River

Mekong River Tonlé Sap River

Phnom Penh, 

Cambodia

0 1 2 3 4 5 KILOMETERS

Chroy Changvar 

peninsula

Koh Norea

peninsula

EXPLANATION

#0 Bench mark

Survey subreach

Direction of flow

Study area

Koh Pich

N34 Bench mark

B

MK.1 Bench mark assac River
6 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

and minimum water levels occur in April. The annual exceedance probability of occurrence of high water levels at Chaktomuk is shown in table 1.

The bed material of the Mekong River at Chaktomuk 

is predominantly sand. The median grain size is approximately 0.3 mm, which generally is classified as medium 

sand (Chaktomuk Project Management Unit and DHI Water 

and Environment, 2002). Upstream from the confluence, 

the bed of the Mekong River becomes more coarse grained 

(sand and gravel), whereas the bed of the Tonlé Sap River is 

composed primarily of silt and fine sand (Chaktomuk Project 

Management Unit and DHI Water and Environment, 2002). 

Suspended-sediment concentration varies in relation to streamflow, with the highest median values found typically in August 

at 0.48 gram/liter (g/L) in conjunction with the rising flows of 

the Mekong River and lowest median values found in December at 0.151 g/L corresponding to declining flows (Chaktomuk 

Project Management Unit and DHI Water and Environment, 

2002). Sediment transport calculations indicated that bed 

material is transported mainly as bed load (Chaktomuk Project 

Management Unit and DHI Water and Environment, 2002). 

The planform shape of the confluence is evolving by a 

process of continual migration towards the south as noted in 

previous studies (Chaktomuk Project Management Unit and 

DHI Water and Environment, 2002). The most pronounced 

feature is the extension of the Chroy Changvar (or Chruy 

Chanvar [Chaktomuk Project Management Unit and DHI 

Water and Environment, 2002]) peninsula, which separates 

the Mekong from the Tonlé Sap River on the northern side of 

the confluence (fig. 3). The elongation rate of the peninsula is 

estimated to be 10 m per year since 1876 (Chaktomuk Project 

Management Unit and DHI Water and Environment, 2002). 

The Tonlé Sap River flows into the northwestern side of the 

confluence and the right bank (west) of the Tonlé Sap River 

is armored with a concrete revetment that has stabilized the 

bank and prevents lateral channel migration. The entrance to 

the Bassac River is on the southern end of the confluence and 

it is bounded by Koh Norea peninsula and Koh Pich island 

(Chaktomuk Project Management Unit and DHI Water and 

Environment, 2002) (fig. 3). The northern tip of Koh Norea 

peninsula, located downstream from the entrance into the Bassac River, is eroding (Chaktomuk Project Management Unit 

and DHI Water and Environment, 2002). Koh Pich is an island 

that is expanding because of placement of dredging spoils. 

Channel dredging is required, because of the deposition of 

silt in the confluence (Phnom Penh Autonomous Port, 2014) 

that otherwise would obstruct the navigation of the confluence and Mekong River allowing large shipping vessels clear 

and safe access to dock at the Phnom Penh Autonomous Port. 

The riverbed at Chaktomuk is characterized by pock-marked 

patterns and deep excavation pits from dredging. The dredge 

spoils serve as an important source of material for land reclamation projects (Chaktomuk Project Management Unit and 

DHI Water and Environment, 2002). Typically, dredge spoils 

from the confluence are deposited near the head of the Bassac 

River on Koh Pich, creating reclamation land that has important economic value (Chaktomuk Project Management Unit 

and DHI Water and Environment, 2002). 

Methods of Investigation

The Mekong, Tonlé Sap, and Bassac Rivers near Phnom 

Penh were surveyed by using a multibeam echosounder 

(MBES) mapping system. The study area was surveyed by a 

team of scientists, technicians, and engineers from the USGS, 

MRC, Phnom Penh Autonomous Port, and the Cambodian 

Ministry of Water Resources and Meteorology. Hydrographic 

surveys were completed during several weeks. 

Description of Equipment

The MBES consists of several components including a 

projector, receiver, link-control unit (LCU), and sonar processor. The entire MBES mapping system includes these MBES 

components, an inertial measurement unit (IMU) and navigation unit, and data collection computers. The MBES mapping 

system and configuration used for the surveys was similar 

to the systems described in Densmore and others (2013), 

Huizinga (2010), and Huizinga and others (2010). 

The MBES used was the RESON SeaBatTM 7125 manufactured in 2008 (RESON A/S, Slangerup, Denmark) operating at a frequency of 400 kilohertz (kHz). The SeaBatTM 7125 

was operated in the equidistance mode during the survey. 

The equidistance mode collects 512 equally spaced depth 

readings each time the projector transmits one sound wave 

(a ping), which has been transformed into beams (table 2). In 

this mode, the beams cover a 128 degrees cross-track swath 

that is focused within a 1 degree along-track area (RESON, 

Inc., 2008). The maximum depth that this MBES can survey 

is 200 m (table 2); however, the depths observed during this 

survey never exceeded this limit. The typical minimum survey 

depth for the SeaBatTM 7125 is 0.5 m; however, the MBES 

was mounted for this survey such that depths of about 1.5 m or 

more were required for standard operation. 

Table 1. Annual exceedance probability of occurrence of water 

levels at Chaktomuk.1

Annual exceedance probability 

of occurrence, in percent

High water level, in meters 

above Ha Tien 1960

50 8.9

20 9.4

10 9.7

5 9.9

3.3 10.0

2 10.1

1

Modified from Chaktomuk Project Management Unit and DHI Water and 

Environment, 2002.
Methods of Investigation 7

The boat used during the hydrographic survey was 15-m 

long with a 3-m beam width and 1-m draft (fig. 4) and was 

selected for its ability to accommodate a work area for several 

computer monitors; the boated needed to maintain a cruising speed of about 2 to 3 meters per second (m/s) during data 

collection. The MBES projector and receiver components 

were mounted on the starboard bow on a pole that allowed 

the MBES projector and receiver components to be rotated 

alongside the boat into the water and locked into surveying 

position about 1 m below the surface (fig. 4C). The MBES 

projector and receiver components were secured in surveying position for the duration of the survey. The projector and 

receiver are each an array of piezo-electric ceramics that 

transform the sound wave into beams through constructive and 

destructive interference. The speed of sound in the water at the 

sonar projector/receiver, as recorded by a sound velocity probe 

(RESON SVP-71) mounted near the projector and receiver 

components, was continuously monitored so depths could be 

accurately calculated for each sounding (RESON, Inc., 2008). 

A separate profiling-sound velocity probe was used to collect 

a speed-of-sound profile throughout the water column at multiple locations in the survey area, and these data were consistent 

with sound velocity data near the surface; therefore, a sound 

velocity profile was not applied to the data. 

The SeaBatTM 7125 MBES is controlled by a sonar processor, which is a high-performance computer that manages 

data flow and signal attributes (transmitted power, ping rate, 

pulse length, and receiver gain). The transmitted power control 

setting increases or decreases the amount of acoustic energy 

transmitted into the water. During the surveys, the acoustic 

power was always set at 220 decibels (dB) and the ping rate 

was typically varied between upstream and downstream passes 

to collect a similar amount of data because the boat moves 

slowly upstream against the current and quickly downstream 

with the current. The ping rate was set to 10 pings per second 

(p/s) for upstream passes and 30 p/s for downstream passes. 

The pulse length controls the length of time the acoustic signal 

is transmitted. Pulse length on the RESON SeaBatTM 7125 can 

range from 33 microseconds (μs) to 300 μs (table 2). A narrow 

pulse length provides high resolution in shallow depths, but a 

wider pulse length provides maximum range with low resolution images. Pulse length was typically set from 40 μs to 60 μs 

for the rivers in the survey area. Receiver gain controls the 

increase in the amplitude applied to the returned sonar signal 

Table 2. Specifications of multibeam-echosounder mapping system equipment.

[kHz, kilohertz; m, meters; °, degrees; Hz, hertz; CW, continuous wave; mm, millimeters; μs, microseconds; RTK, real-time kinematic; GNSS, Global Navigation Satellite System; m, meters; m/s, meters 

per second; ppm, part per million; RMS, root mean square]

RESON SeaBatTM 7125 specifications (RESON, Inc., 2008)

Frequency 400 kHz

Maximum depth 200 m

Swath coverage 128°

Beam width 0.5° × 1°

Number of beams 256/512 Equi-angle or 512 equidistant

Maximum update rate 50 Hz

Wave form Gated CW

Depth resolution 5 mm

Pulse length 33 to 300 μs

System control 7-P Processor unit

Mounting angle 20 to -20°

POS MV WaveMaster system (Applanix Corporation, 2006) with RTK GNSS corrections

Pitch and roll accuracy 0.02°

Heave accuracy 0.05 m

Heading accuracy 0.06°

Positional accuracy 0.02–0.09 m

Velocity accuracy 0.05 m/s

Trimble® R8 GNSS receivers (Trimble® Navigation Limited, 2009)

Horizontal accuracy 10 mm + 1 ppm RMS

Vertical accuracy 20 mm + 1 ppm RMS
8 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

in addition to the calculated time-varied gain (TVG). Receiver 

gain was typically set for 30 to 40 dB. 

The IMU and navigation system used with the MBES 

(fig. 4B) was an Applanix Position Orientation Solution for 

Marine Vessels (POS MVTM) WaveMaster system (Applanix 

Corp., Richmond Hill, Ontario, Canada). The POS MVTM

system consists of two Trimble Zephyr Global Positioning 

System (GPS) antennas (Trimble Navigation, Ltd., Sunnyvale, Calif.), the IMU, and a POS Computer System that 

is configured and monitored through POSView Controller 

software. The GPS antennas were mounted on the rail of the 

observation deck about 4 m above the MBES projector and 

receiver (fig. 4A). The IMU consists of three linear accelerometers and three solid-state gyroscopes arranged in a triaxial 

orthogonal array to sense acceleration and angular motion 

in all three axes, and was located at the center of the bow of 

the boat about 7.5 m forward of the boat’s center of gravity (fig. 4B). Data from the GPS antennas and from the IMU 

were combined by the POS MVTM system to provide position, 

roll and pitch angles, true heading, and real-time heave of 

the boat to enable correction of the MBES data. A Real-Time 

Kinematic (RTK) GPS/Global Navigation Satellite System 

(GPS/GNSS) radio link to a base station (Trimble® R8 GNSS 

receiver [Trimble® Navigation Limited, 2009]) located on a 

bench mark provided additional correction information. The 

GPS/GNSS Azimuth Measurement Subsystem (GAMS) algorithm of the POS MVTM and RTK solution was used during 

the hydrographic survey; therefore, the accuracy of the roll 

Figure 4. A multibeam-echosounder mapping system. A, View of front of boat with the global positioning system antennas, navigation 

monitor, and sonar mount. B, View forward from the bow with the inertial measurement unit (IMU) and sonar mount. C, View of the 

stowed sonar head (rotated approximately 180 degrees from deployed position) with the projector, receiver, and sound velocity probe. 

D, View of main cabin interior with the computer system setup including sonar and HYPACK® monitors and weather-resistant computer 

lock box.

Navigation

monitor

Sonar 

mount

Global positioning 

system antennas

A B

Inertial 

measurement 

unit (IMU)

Sonar mount

C Projector Receiver

Sound 

velocity probe

D Sonar monitor

HYPACKTM monitor

Weatherresistant 

computer 

lock box
Methods of Investigation 9

and pitch measured was 0.02°, heave was 0.05 m, heading was 

0.06°, position was 0.02 to 0.09 m, and velocity was 0.05 m/s 

(Applanix Corporation, 2006) (table 2). When GPS/GNSS 

reception outages occurred, typically near shorelines and 

under bridges, the POS MVTM provided the sole source of boat 

position information, which caused a slight degradation of 

accuracy for those periods.

The high-resolution MBES system required detailed 

documentation of sensor locations, timing differences, and 

instrument calibrations to achieve a high level of precision 

and accuracy. The positions of each sensor were precisely 

measured in relation to all others using a total-station survey 

instrument. Timing between the MBES instruments was synchronized using a pulse-per-second message sent to the sonar 

from the POS MVTM. 

All the sounding, navigation, and motion data were 

combined, saved, and displayed using the HYPACK®/

HYSWEEP® software (HYPACK, Inc., 2007a and 2007b) and 

were continuously monitored on a laptop computer during data 

collection (fig. 4D). An operator monitored data streams and 

oversaw navigation in real-time while concurrently refining 

sonar settings. A separate monitor, displaying the navigation 

software output, was positioned at the boat pilot’s station so 

that survey lines could be followed and survey plan boundaries, orthophotographs, and charts could be viewed while data 

were collected (fig. 4D). 

Hydrographic Survey Methods

The hydrographic survey was completed using a highresolution MBES mapping system installed on a modified 

boat. A brief overview of the various component parts of the 

MBES are given in the “Description of Equipment” section of 

this report. The field procedures and data-processing procedures used to complete the survey are described in the following sections.

Field Procedures

The goal of the study was to survey the riverbed elevations at Chaktomuk to help develop visualization and modeling tools and to provide a dataset for comparison with 

previous and future surveys. To achieve this goal, the survey 

included as much of the region near the confluence as was 

practical in the time available. The survey area, generally 

centered on the confluence of the rivers, was divided into six 

survey subreaches (fig. 3). Each subreach was small enough 

to be surveyed within 3 days to minimize discontinuity in 

survey data caused by ongoing processes, such as water-level 

changes, bedform translation, and anthropogenic changes. The 

location and extent of each subreach were based on assumptions of boat speed, river width and depth, and distance to a 

survey-control bench mark (GNSS base station). 

The first subreach to be surveyed was on the Mekong 

River at the downstream end of the study area. The orientation 

of the MBES sonar mount was not changed during surveying to avoid errors related to different mounting positions. 

The boat pilot followed streamwise (parallel to the direction 

of flow) survey lines that were projected onto a navigation 

display to obtain broad spatial coverage along each river. Gaps 

between parallel swaths were permitted, especially in wide, 

shallow areas where swath widths were limited by depth. 

MBES settings such as power, gain, and filter windows were 

adjusted based on depth, quality of received signal, and noise 

(random fluctuation in the signal). 

At the beginning of each survey day, a GPS/GNSS base 

station was deployed on one of two bench marks (fig. 3) with 

known geographic coordinates. During data collection, the 

World Geodetic System of 1984 (WGS 84) ellipsoid was used 

to reference height because elevation referenced to the local 

vertical datum, Ha Tien 1960, was unknown for either of the 

bench marks. Although real-time corrections to GPS/GNSS 

positions were applied to data points obtained during the 

survey, the accuracy of the survey points was dependent on 

the accuracy of the assumed coordinates of the bench marks. 

After the survey was completed, the geodetic position of the 

bench marks was revised based on an average of National 

Geodetic Survey Online Positioning User Service (OPUS; 

National Geodetic Survey, 2014) solutions computed from 

data collected at each bench mark during the study (table 3). 

A correction based on the refinement of the bench mark location was applied to the survey data during post-processing. A 

bench mark, designated MK.1 located near the downstream 

end in the survey area, was used for the most downstream 

survey subreach, and the Chroy Changvar bench mark, designated N34, near the upstream end of the study area (fig. 3 and 

table 3) was used for the rest of the survey subreaches. 

Data-Processing Procedures

Using MBES data required software and several steps 

that were capable of processing large datasets. The USGS 

used three different software packages to process the MBES 

data: (1) HYPACK®/HYSWEEP® software, primarily for data 

collection; (2) CARIS HIPS and SIPSTM and Bathy DataBASE: Base EditorTM software (CARIS, 2012a and 2012b), 

primarily for post-processing; and (3) Esri’s ArcMap software 

(Esri, 2012) for additional post-processing, including elevation 

adjustments and visualization of mapping results. 

First, all offset values (distances between MBES components used by the software) and calibration values were 

either determined or evaluated, and the values were applied 

to the dataset before it was edited. Calibration values were 

determined from processing data collected specifically for 

the calibration test. This process is described in the “Quality 

Assurance and Quality Control” section of this report. Offset 

values were determined for the configuration of the boat for 

data collection and did not change as long as mounting points 

remained unaltered; however, the surveyor verified that all 

offsets were entered correctly for each dataset. 
10 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

Second, water-surface heights were reviewed to ensure 

that accurate corrections were applied to the depth measurements, which, in turn, were used to calculate the height of 

riverbed above the reference ellipsoid (WGS 84). If watersurface heights were not deemed to be sufficiently accurate 

(evaluated if a point was collected using fixed GPS/GNSS 

solution) throughout the dataset, then a time series of watersurface profiles were created by selecting representative 

upstream and downstream water-surface heights from each 

survey dataset. The water-surface profiles were used to spatially interpolate the water surface for successive intervals for 

the time of the survey. 

After water-surface heights were determined and applied 

to the dataset, the bathymetric soundings were filtered to 

remove data that did not agree within 0.01 m of the riverbed 

height collected by adjacent beams. Filters removed spikes 

or narrow peaks likely caused by interference with the sonar 

signal. Data points flagged as poor quality because they were 

collected at low signal strength and (or) because they were not 

collinear in elevation/depth were removed. Data points from 

the extreme outside edges of the swath were removed because 

random fluctuations often are present in the outermost beams. 

After filters were applied, each swath was reviewed in graphic 

displays, and remaining values that were determined by a 

hydrographer to be erroneous were edited or deleted from the 

dataset.

The CARIS HIPS and SIPSTM implementation of the 

Combined Uncertainty and Bathymetry Estimator (CUBE) 

algorithm (Calder and Wells, 2007) was used to create a 

riverbed surface model based on the hydrographic dataset, 

which consisted of more than 53 million individual points. 

The CUBE algorithm uses a data reduction technique that 

estimates the depth and a confidence interval for each node 

of a gridded surface model based on the points in the dataset. 

Therefore, the gridded surface model is a “smoothed” representation of the bathymetric data points and contains less 

variance. The surface model was exported in a geospatial 

raster format—grids with 0.5-m by 0.5-m cells. Each cell 

value represents an estimate of the most likely riverbed height 

associated with the area inside the cell. On average, about 

48 data points were used to estimate the most likely riverbed 

height in each cell.

Next, the coordinate values in the hydrographic dataset 

were measured in reference to the WGS 84 ellipsoid, but the 

local tidal datum used to reference elevation above mean sea 

level is Ha Tien 1960. The vertical separation between Ha 

Tien 1960 and WGS 84 ellipsoid is about 10 m in the Lower 

Mekong River Basin (Falke, 2009); however, the separation 

Table 3. Location and location checks of bench marks used for hydrographic surveys of Chaktomuk near 

Phnom Penh, Cambodia, 2012.

[WGS 84, World Geodetic System of 1984; OPUS, Online Positioning User Service]

Date surveyed Bench-mark 

identifier

Source of 

coordinates

Coordinates referenced to WGS 84

Longitude Latitude Ellipsoid height, 

in meters

1/6/2006 MK.1 Previous survey1 104°58ʹ04.2665ʺ 11°32ʹ22.9218ʺ -1.333

3/27/2012 MK.1 OPUS solution 104°58ʹ04.2858ʺ 11°32ʹ22.9164ʺ -1.625

4/20/2012 MK.1 OPUS solution 104°58ʹ04.2879ʺ 11°32ʹ22.9167ʺ -1.705

4/21/2012 MK.1 OPUS solution 104°58ʹ04.2862ʺ 11°32ʹ22.9174ʺ -1.747

4/23/2012 MK.1 OPUS solution 104°58ʹ04.2863ʺ 11°32ʹ22.9166ʺ -1.756

3/27–4/23/2012 MK.1 OPUS average 104°58ʹ04.2865ʺ 11°32ʹ22.9168ʺ -1.736

1993 N34 Previous survey2 104°56ʹ18.4424ʺ 11°35ʹ16.5024ʺ 5.774

4/24/2012 N34 OPUS solution 104°56ʹ18.4602ʺ 11°35ʹ16.4968ʺ 5.609

4/25/2012 N34 OPUS solution 104°56ʹ18.4605ʺ 11°35ʹ16.4958ʺ 5.619

4/26/2012 N34 OPUS solution 104°56ʹ18.4608ʺ 11°35ʹ16.4958ʺ 5.590

4/27/2012 N34 OPUS solution 104°56ʹ18.4603ʺ 11°35ʹ16.4960ʺ 5.606

4/28/2012 N34 OPUS solution 104°56ʹ18.4610ʺ 11°35ʹ16.4958ʺ 5.581

4/30/2012 N34 OPUS solution 104°56ʹ18.4609ʺ 11°35ʹ16.4955ʺ 5.648

5/1/2012 N34 OPUS solution 104°56ʹ18.4623ʺ 11°35ʹ16.4954ʺ 5.695

5/2/2012 N34 OPUS solution 104°56ʹ18.4620ʺ 11°35ʹ16.4955ʺ 5.642

4/24–5/2/2012 N34 OPUS average 104°56ʹ18.4610ʺ 11°35ʹ16.4958ʺ 5.624

1

Cambodian Ministry of Public Works and Transport, written commun., 2006.

2

Institut Géographique National Français, written commun., 1993.
Methods of Investigation 11

between the two datums is poorly documented in the study 

area. Global geoid models of the earth such as the WGS 84 

Earth Gravitational Model 2008 (EGM2008) are surfaces 

referenced to the global mean ocean surface elevation. In contrast, a local tidal datum is referenced to the long-term average 

of tides at a specific location (or locations). Heights referenced to the WGS 84 ellipsoid can be converted to elevations 

referenced to a local tidal datum through a two-step process if 

the separation of the local tidal datum from a geoid model is 

known. 

For this hydrographic survey, conversion of the WGS 84 

ellipsoid heights to elevations above Ha Tien 1960 was 

achieved by first converting the ellipsoid heights to orthometric heights referenced to EGM2008, and then from orthometric 

heights above EGM2008 to heights above Ha Tien 1960 based 

on separation between Ha Tien 1960 and EGM2008. 

For the USGS hydrographic survey of Chaktomuk in 

2012, heights above the WGS 84 ellipsoid were converted to 

orthometric heights referenced to EGM2008 using the following equation:

H=h˗N (1)

where

H is the orthometric height, in meters; 

h is the WGS 84, ellipsoid height, in meters; 

and

N is the EGM2008, geoid height, in meters.

Geoid heights were computed for each point on a 100-m 

by 100-m grid covering the entire study area using software 

developed by the National Geospatial-Intelligence Agency 

(National Geospatial-Intelligence Agency, 2013). In general, 

geoid heights trended from about -12.38 m in the southeast 

part of the survey to about -12.81 m in the northwest. The 

geoid grid then was subtracted from the hydrographic grids 

to convert the vertical reference from the WGS 84 ellipsoid 

to orthometric heights based on EGM2008. Previous GPS 

surveys of a second-order vertical control point in Phnom 

Penh (not used in this survey) indicated that Ha Tien 1960 is 

0.76 m lower than EGM2008 (F.S. Seebacher, Mekong River 

Commission, written commun., 2014). Thus, a separation 

offset of 0.76 m was assumed to be valid for the study area 

and was subtracted from the orthographic grids referenced 

to EGM2008 to create hydrographic grids referenced to 

Ha Tien 1960.

Last, hydrographic maps of the survey subreaches in the 

Mekong, Tonlé Sap, and Bassac Rivers were produced from 

the 0.5-m by 0.5-m grids referenced to Ha Tien 1960. The 

riverbed elevation associated with the center of each grid cell 

was exported as an (x-y-z) text file for publication.

Quality Assurance and Quality Control

Two primary methods were used for quality assurance 

and quality control (QA/QC) during collection and processing of data with the MBES mapping system: operator QA/QC 

during data collection and patch tests. During collection of 

hydrographic data, the MBES system operator was continuously checking the data being logged for poor soundings (such 

as those that had low echo intensity or were not collinear with 

the surrounding data) and adjusting settings on the system to 

ensure that the best possible data were collected. The MBES 

operator also compared data collected from overlapping 

swaths and subreaches to ensure that the data being collected 

were well correlated with those from previous passes.

Patch tests are short survey lines located in specifically 

selected test areas, and when surveyed in parallel or perpendicular directions, the results allow timing and determination 

of orientation offsets of the MBES with respect to the IMU. 

Patch-test data were collected in HYPACK®/HYSWEEP®

software following the same procedures that were used to collect data that were not used for patch tests, but data from these 

specific survey lines were further evaluated using HYPACK®/

HYSWEEP® software to determine latency, or timing difference between the MBES and the POS MVTM positioning 

data, and angular offset values. In addition, patch tests were 

used to determine the angular offsets of the MBES projector/

receiver with respect to the IMU. These angular offsets are 

referred to as roll, pitch, and yaw. Uncorrected latency, pitch, 

and yaw offsets appear as horizontal offsets in the collected 

data, whereas uncorrected roll offsets appear as vertical and 

horizontal offsets in the data. Further details of patch tests are 

documented in Densmore and others (2013) and HYPACK, 

Inc. (2007b). 

The latency test was used to determine a timing offset 

(Δt) between the MBES mapping system and the positioning 

or GPS component of the POS MVTM. Timing offsets were 

determined by collecting data on the same survey line over 

the riverbed slope or bedform feature in the same direction, 

but at two different speeds. The latency test completed for this 

survey determined that the latency (Δt) was zero.

A roll test was used to determine the angular offset of the 

sonar projector/receiver alignment with respect to the IMU 

orientation along the longitudinal axis of the boat. To determine the roll-angle offset (α), data were collected over a flat 

area on one survey line; data were collected twice while the 

boat moved in opposite directions. The results of these roll 

tests indicated that the roll-angle offset was 2.8 degrees (away 

from the boat).

The pitch test was used to determine the angular offset 

of the MBES transducer projector/receiver alignment with 
12 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

respect to the IMU orientation in the lateral axis or along the 

track direction of the boat. To determine the pitch-angle offset 

(β), one line was surveyed twice over a slope or bedform 

feature; data were collected while the boat traversed the line in 

opposite directions. The pitch test for this survey determined 

that the pitch-angle offset along the lateral axis of the boat was 

12.8 degrees.

The yaw test was used to determine the angular offset 

of the MBES projector/receiver alignment with respect to 

the IMU orientation about the vertical axis. To determine the 

yaw-angle offset (δ), two parallel lines were surveyed while 

the boat moved in the same direction over a slope or bedform 

feature. The yaw test for this survey determined that the yawangle offset was 0.0 degrees.

Overall vertical accuracy of the hydrographic data presented in this report is estimated to be plus or minus 0.2 m. 

This estimate is based on equipment specifications, evaluation of overlap data, and previous data collection with this 

equipment. Accuracy of hydrographic survey data is typically 

challenging to evaluate because there are few underwater areas 

with known elevation. However, the quality of hydrographic 

surveys is often described by uncertainty estimates called total 

propagated uncertainty (TPU) (CARIS, 2011). Calculations of 

TPU were completed for the hydrographic survey data. The 

maximum TPU was 0.33 m, and the mean and median were 

0.18 m (table 4). The percentage of bathymetry points with 

TPU values less than 0.25 m and 0.20 m was 99.9 percent and 

94.2 percent, respectively. 

Hydrographic Survey Maps

Using the fully processed 0.5-m resolution dataset, 

detailed hydrographic maps of the Mekong, Tonlé Sap, and 

Bassac Rivers were produced that document the riverbed 

elevations, as surveyed April 21–May 2, 2012, and referenced 

to Ha Tien 1960. The six subreaches of the study area (fig. 3) 

were combined into four sections (fig. 5) to show higherresolution maps and to delineate areas for statistical summaries. Three of the four sections primarily depict the Mekong 

River (figs. 6–8), whereas the forth section includes the Tonlé 

Sap River from its mouth to the Chroy Changvar Bridge 

(fig. 9). Riverbed features (such as dunes, shoals, and the 

effects of sediment mining) observed during data collection 

are visible on the maps. Statistical summaries of the surveyed 

riverbed elevations are presented by section of the study area 

(table 5 and fig. 10). Riverbed elevations surveyed for the 

entire survey area ranged from 24.08 m below to 1.54 m above 

Ha Tien 1960. 

All surveys were completed at relatively low water levels. Water-surface elevations during 2012 at selected streamflow-gaging stations ranged from 1.54–8.60 m on the Mekong 

River and 1.04–7.69 m on the Tonlé Sap River (table 5) 

(Mekong River Commission, 2014b). During the hydrographic 

surveys, the mean water-surface elevations ranged from 

1.85–1.97 m on the Mekong River and the mean water-surface 

elevation on the Tonlé Sap River was 1.13 (table 5); these 

values are at the low end of the annual water-surface elevation 

range at these streamflow-gaging stations. No nearby streamflow-gaging stations were available for the Bassac River. 

Table 4. Summary by section of study area of total propagated uncertainty for the hydrographic survey of Chaktomuk near Phnom 

Penh, Cambodia, April 21–May 2, 2012.

[TPU, total propagated uncertainty]

Section of study area

Maximum 

value of 

TPU, in 

meters

Mean 

TPU, in 

meters

Median 

value of 

TPU, in 

meters

Standard 

deviation 

of TPU 

values, in 

meters

Bathymetry points with TPU values less than 

indicated value, in percent

 0.25 meter 0.20 meter 0.15 meter 0.10 meter 

1

Mekong River, upstream from 

confluence (fig. 6) 

0.23 0.19 0.18 0.01 100.0 97.09 0.3 0

Mekong River, at confluence (fig. 7) 0.29 0.18 0.18 0.01 99.9 94.49 2.4 0.1

1

Mekong River, downstream from 

confluence (fig. 8)

0.25 0.18 0.18 0.02 100.0 92.31 4.3 0.2

Tonlé Sap River (fig. 9) 0.33 0.18 0.18 0.02 99.5 92.84 4.2 0.7

Entire survey area

0.33 0.18 0.18 0.01 99.9 94.2 2.2 0.1

1

Subreaches were combined for these final 4 sections (figs. 3 and 5).
Hydrographic Survey Maps 13

Figure 5. Hydrographic survey of Chaktomuk, the confluence of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, 

Cambodia, April 21–May 2, 2012.

490,000 492,000 494,000 496,000 498,000 500,000

1,276,000 1,278,000 1,280,000 1,282,000

0 1.0 2.0 KILOMETERS

0 0.5 1.0 MILE

Mekong River

Mekong River

Tonlé Sap River

Bassac River

Figure 6

Phnom Penh

Figure 8

Figure 7

Figure 9

N34 Bench mark

#MK.1 Bench mark

#

EASTING, in meters

NORTHING, in meters

Base from Esri Imagery Layer Package, 2012

Horizontal coordinate information referenced to the 

Universal Transverse Mercator projection, zone 48 North

Mekong River

Three sections of the Mekong River (fig. 5) were surveyed April 21–May 2, 2012, and included the section extending upstream from the southern end of the Chroy Changvar 

peninsula (fig. 6), a section at the confluence of the Mekong, 

Tonlé Sap and Bassac Rivers (fig. 7), and a section downstream from the confluence (fig. 8). 

The upstream section of the Mekong River was surveyed 

April 30–May 2 (fig. 6). The upstream section was about 4 km 

long and about 1,600 m wide at the most upstream part of the 

section and then narrowed to about 1,000 m in the downstream 

part of the section. Riverbed elevations were less than -18 m 

in the center of the channel for most of this section (fig. 6). 

Compared with the downstream section surveyed for this 

study (fig. 8), this upstream section is narrow and deep with 

50 percent of the surveyed riverbed area at an elevation less 

than -12.5 m (fig. 10A). 

The middle section at the confluence of the Mekong, 

Tonlé Sap, and Bassac Rivers was surveyed on April 24–25. 

The middle section was about 2 km long and the channel was 

approximately 1,500 m wide (fig. 7). The mouth of the Tonlé 

Sap River and the head of the Bassac River distributary were 

included in this section of the survey. Areas of sediment mining were present near the downstream end of Chroy Changvar 

peninsula and extended about 1,500 m downstream. Riverbed 

elevations where sediment was mined were as much as 8 m 

lower than the riverbed elevations in the channel immediately 

adjacent to areas that were not mined (fig. 7). The riverbed 

elevations in this section were not as deep as in the upstream 

section, with 50 percent of the riverbed area at an elevation 

less than -9 m (fig. 10B). 

The Mekong River section downstream from the confluence was surveyed on April 21–23 and April 27–28. The 

downstream section was about 3.6 km long and the channel 

ranged from about 1,300 to 1,800 m wide (fig. 8). The effects 

of sediment mining are visible near the downstream end of the 

section, which is locally 12 m lower than the typical riverbed elevations adjacent to the mined area (fig. 8). Two scour 

holes near the right bank also were evident: (1) about 600 m 

downstream from the Bassac River was a smaller scour hole 

where the surveyed riverbed elevations were about 11 m less 

than typical riverbed elevations adjacent to the scour hole, and 

(2) downstream from a protrusion in the bank (about 2,000 m 

downstream from the Bassac River) was a larger scour hole 

where surveyed riverbed elevations were about 16 m less than 
14 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

Figure 6. Hydrographic survey of the Mekong River upstream from the confluence of the Mekong, Tonlé Sap, and 

Bassac Rivers near Phnom Penh, Cambodia, April 30–May 2, 2012.

#0

493,000 493,500 494,000 494,500 495,000 495,500

1,279,500 1,280,000 1,280,500 1,281,000 1,281,500 1,282,000 1,282,500 1,283,000

Mekong River

EXPLANATION

# Bench mark

Elevation, in meters

2

0

-3

-6

-9

-12

-15

-18

-21

-24

Flow

m

r

0 0.25 0.5 0.75 1.0 KILOMETER

0 0.25 0.5 MILE

N34 Bench mark

EASTING

NORTHING

Chroy Changvar

peninsula

Base from Esri Imagery Layer Package, 2012

Horizontal coordinate information referenced to the 

Universal Transverse Mercator projection, zone 48 North

Vertical coordinate information referenced to Ha Tien 1960
Summary 15

typical riverbed elevations adjacent to this scour hole (fig. 8). 

In general, the riverbed elevations in this third section of the 

Mekong River were the shallowest of all surveyed sections, 

with 50 percent of the riverbed area at an elevation less than 

-5 m (fig. 10C). 

Bassac River

Approximately 700 m of the Bassac River was surveyed 

on May 2 on as part of the Mekong River middle-section 

survey (fig. 7). Water depths in the Mekong River were generally much shallower near the bifurcation at the head of the 

Bassac River; however, one area of deep water was present 

near the left bank of the Bassac River (at Koh Norea peninsula) near an area where active bank erosion was evident. At 

the time of the survey, construction was occurring on the west 

bank of the Mekong River at Koh Pich, and structures associated with that activity had diverted flow away from the Bassac 

River (fig. 7). About 100 m downstream from the head of the 

Bassac River, riverbed elevations in the Bassac River were 

typically about -9 m. 

Tonlé Sap River

On April 26, 2012, the Tonlé Sap River was surveyed 

from its mouth to the Chroy Changvar Bridge; the section is 

about 3.5 km long and the channel was typically 300–400 m 

wide along the surveyed section (fig. 9). At the time of the survey, the Tonlé Sap River was flowing into the Mekong River. 

The hydrographic map shows that depths on the Tonlé Sap 

River were 2 to 3 times shallower than on the section of the 

Mekong River upstream from the confluence, with 50 percent 

of the riverbed area surveyed on the Tonlé Sap River at an 

elevation less than -5 m (fig. 10C ). 

Summary

Forecast Mekong is a project led by the U.S. Geological Survey (USGS) that provides data integration; predictive 

environmental, natural, and water resource models; and scientific visualization tools to help decision makers. A component 

of this effort was a hydrographic survey of Chaktomuk, the 

confluence of the Mekong, Tonlé Sap (also spelled Tônlé Sab), 

and Bassac Rivers near Phnom Penh, Cambodia. The USGS, 

in cooperation with the U.S. Department of State, Mekong 

River Commission (MRC), Phnom Penh Autonomous Port, 

and the Cambodian Ministry of Water Resources and Meteorology, surveyed this area to collect data that could improve 

navigation and shipping safety and provide baseline scientific 

data to document natural and anthropogenic disturbances and 

geomorphic change in the confluence area. The hydrographic 

survey was completed in 2012 and supports the goals of Forecast Mekong by providing data to help visualize the confluence and aid the development of planning tools.

The Mekong River begins in the Tibetan Plateau of 

China. The river flows southeast for approximately 4,800 kilometers through the countries of China, Myanmar (Burma), 

Laos, Thailand, Cambodia, and Vietnam, and discharges into 

the South China Sea. The river basin area is approximately 

795,000 square kilometers. The mean annual discharge is 

approximately 14,500 cubic meters per second and the annual 

runoff is approximately 457 cubic kilometers. The Mekong 

River typifies large, tropical monsoonal rivers, with large 

mean annual discharge distributed in conjunction with welldefined wet and dry seasons. The wet season of the monsoon 

cycle typically lasts from May until early October, with the 

greatest precipitation usually falling in August and September. 

The confluence of the Mekong, Tonlé Sap, and Bassac 

Rivers is known as Chaktomuk. For most of the year, the 

Tonlé Sap River flows into the Chaktomuk, and the Bassac 

River flows out of the Chaktomuk to the South China Sea. 

When stage is high in the Mekong River the Tonlé Sap River 

flows to Tonlé Sap Lake. Conversely, when stage is low in the 

Mekong River the Tonlé Sap River flows to Chaktomuk and 

the Mekong River. The Tonlé Sap Lake serves as a floodstorage reservoir for the Mekong River and supports important 

wetlands, fisheries, and rice-growing areas. Sedimentation or 

other geomorphic and hydrologic alterations at Chaktomuk 

could alter this unique and important hydrologic system. 

The Mekong, Tonlé Sap, and Bassac Rivers near Phnom 

Penh were surveyed by using a multibeam echosounder 

(MBES) mapping system, composed of a RESON SeabatTM

7125 MBES, an inertial measurement and navigation unit, 

data collection computers, and a Real-Time Kinematic (RTK) 

Global Navigation Satellite System (GNSS) base station. The 

survey area was divided into subreaches, and each subreach 

was surveyed within 3 days to minimize discontinuity in 

survey data caused by ongoing processes, such as water-level 

changes, bedform translation, and anthropogenic changes. 

The survey was referenced to two existing bench marks 

with known geographic coordinates by establishing a GNSS 

base station on the bench marks each day and using the 

real-time corrections from the base station to correct the boat 

navigation data. During data collection, the World Geodetic 

System of 1984 (WGS 84) ellipsoid was used as a to reference 

riverbed height. The hydrographic survey data were converted 

from WGS 84 ellipsoid heights to elevations above Ha Tien 

1960 during postprocessing.
16 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

Figure 7. Hydrographic survey of the Mekong River at the confluence of the Mekong, Tonlé Sap, and Bassac Rivers near 

Phnom Penh, Cambodia, April 24–April 25, 2012.

492,500 493,000 493,500 494,000 494,500 495,000 495,500

1,276,000 1,276,500 1,277,000 1,277,500 1,278,000 1,278,500 1,279,000 1,279,500

0 0.25 0.5 0.75 KILOMETER 1

0 0.25 0.5 MILE

Mekong River

EXPLANATION

Elevation, in meters

2

0

-3

-6

-9

-12

-15

-18

-21

-24

Flow

Flow

Sediment mining

Land fill and development

Chroy Changvar

peninusla

Koh Pich

Bassac River

Koh Norea

peninsula

Active bank erosion

m

m

EASTING

NORTHING

Tonlé Sap River

Base from Esri Imagery Layer Package, 2012

Horizontal coordinate information referenced to the 

Universal Transverse Mercator projection, zone 48 North

Vertical coordinate information referenced to Ha Tien 1960
Summary 17 Figure 8. Hydrographic survey of the Mekong River downstream from the confluence of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, April 21–April 23 and April 27–April 28, 2012.

#0

494,500 495,000 495,500 496,000 496,500 497,000 497,500 498,000 498,500

1,278,000 1,277,500 1,277,000 1,276,500 1,276,000 1,275,500

0 0.5 1 0.25 0.75 KILOMETER0 0.25 0.5 MILE

Mekong River

EXPLANATION

#0 Bench mark

Elevation, in meters

2 0 -3 -6 -9 -12 -15 -18 -21 -24

Sediment mining

Bassac

River

MK.1 Bench mark

Scour hole

EASTING

NORTHING

Flow m

Base from Esri Imagery Layer Package, 2012Horizontal coordinate information referenced to the Universal Transverse Mercator projection, zone 84 NorthVertical coordinate information referenced to Ha Tien 1960
18 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

Figure 9. Hydrographic survey of the Tonlé Sap River upstream from the confluence of the Mekong, Tonlé Sap, and 

Bassac Rivers near Phnom Penh, Cambodia, April 26, 2012.

#0

491,000 491,500 492,000 492,500 493,000 493,500

1,278,000 1,278,500 1,279,000 1,279,500 1,280,000 1,280,500 1,281,000

0 0.25 0.5 0.75 KILOMETER 1

0 0.25 0.5 MILE

Mekong River

EXPLANATION

#0 Bench mark

Elevation, in meters

2

0

-3

-6

-9

-12

-15

-18

-21

-24.08

Flow

m

Chroy Changvar

peninsula

Tonlé Sap River

Flow

m N34 Bench mark Chroy Changvar bridge Chroy Changvar Bridge

EASTING

NORTHING

Base from Esri Imagery Layer Package, 2012

Horizontal coordinate information referenced to the 

Universal Transverse Mercator projection, zone 84 North

Vertical coordinate information referenced to Ha Tien 1960
Table 5. Summary of riverbed elevations surveyed at Chaktomuk, Cambodia, April 21–May 2, 2012, and water-surface elevations at two 

streamflow-gaging stations.

[All elevations are referenced to Ha Tien 1960; km2, square kilometers; m, meters; WSE, water surface elevation; km, kilometers]

Section of study area Survey period

Area 

surveyed 

(km2

)

Minimum 

riverbed 

elevation 

observed 

(m)

Maximum 

riverbed 

elevation 

observed 

(m)

Mean 

WSE 

(m)

Maximum 

WSE 

(m)

Minimum 

WSE 

(m)

Maximum 

WSE 

(m)

Minimum 

WSE 

(m)

Survey period January–

December, 2012

Mekong River, upstream 

from confluence (fig. 6)

April 30–May 2, 

2012

3.70 -22.96 1.00 1

1.89 12.18 11.55 18.60 11.54

Mekong River, at confluence (fig. 7)April 24–25, 

2012

3.51 -24.08 1.54 1

1.97 12.18 11.77 18.60 11.54

Mekong River, downstream from confluence 

(fig. 8)

 April 21–23; 

April 27–28, 

2012

4.51 -23.27 1.51 1

1.85 12.41 11.56 18.60 11.54

Tonlé Sap River (fig. 9) April 26, 2012 1.15 -15.18 1.01 2

1.13 21.14 21.12 27.69 21.04

Entire survey area

April 21–May 2, 

2012

12.87 -24.08 1.54 1

1.89 12.18 11.55 18.60 11.54

1

Water-surface elevation of the Mekong River observed at Mekong River Commission streamflow-gaging station 019802 at Kompong Cham, approximately 

77 km upstream from the study area (Mekong River Commission, 2014b).

2

Water-surface elevation of the Tonlé Sap River observed at Mekong River Commission streamflow-gaging station 020102 at Prek Kdam, approximately 

30 km upstream from the study area (Mekong River Commission, 2014b).

Summary 19

The quality of hydrographic surveys was described by 

uncertainty estimates called total propagated uncertainty 

(TPU). Results of TPU calculated for the hydrographic survey 

data indicated that the maximum TPU was 0.33 m, the mean 

and median were 0.18 m, and 99.9 percent of TPU values were 

less than 0.25 m.

Detailed hydrographic maps of Mekong, Tonlé Sap, and 

Bassac Rivers were produced that document the riverbed 

elevations surveyed April 21–May 2, 2012. The surveyed area 

of the Mekong River included a 4-km reach upstream from 

the confluence, a 2-km reach at the confluence, and a 3.6-km 

reach downstream from the confluence. In addition, 0.7 km 

of the Bassac River and 3.5 km of the Tonlé Sap River (from 

the confluence to Chroy Changvar Bridge) were surveyed. 

Riverbed features (such as dunes, shoals, and the effects of 

sediment mining, which were observed during data collection) are visible on the hydrographic maps. All surveys were 

completed at low water levels during 2012, as indicated by 

selected Mekong River Commission streamflow-gaging stations. Riverbed elevations surveyed ranged from -24.08 m to 

1.54 m above Ha Tien 1960. 
20 Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012

Figure 10. Frequency distributions of the areas of riverbed elevations surveyed at Chaktomuk, the confluence of the Mekong, 

Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, April 21–May 2, 2012. A, Mekong River upstream from the confluence. 

B, Mekong River at the confluence with the Tonlé Sap and Bassac Rivers. C, Mekong River downstream from the confluence. 

D, Tonlé Sap River.

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

 100

A. Mekong River upstream from the confluence

D. Tonlé Sap River 

0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

50,000

0

20,000

40,000

60,000

80,000

100,000

120,000

-25 -20 -15 -10 -5 0 5 -25 -20 -15 -10 -5 0 5

 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

 100

 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

 100

 0

 10

 20

 30

 40

 50

 60

 70

 80

 90

 100

Riverbed area, in square meters

0

2,000

4,000

6,000

8,000

10,000

12,000

Cumulative percent of survey-grid cells lower than indicated elevation

C. Mekong River downstream from the confluence

Elevation, in meters above the Ha Tien Datum of 1960

Cumulative 

percent 

B. Mekong River at the confluence with Tonlé Sap 

 and Bassac Rivers

Cumulative 

percent

Cumulative 

percent 

Cumulative 

percent 

References Cited

Adamson, P.T, Rutherford, I.D., Peel, M.C., and Conlan, I.A., 

2009, The hydrology of the Mekong River, in Campbell, 

I.C., eds., The Mekong—Biophysical environment of an 

international river basin: Amsterdam, Elsevier, p. 53–75. 

Applanix Corporation, 2006, POS-MVTM V4 user guide, 

rev. 0: Richmond Hill, Ontario, Applanix, PUBS–MAN–

000513, 369 p.

Calder, B.R, and Wells, D.E., 2007, CUBE user’s manual, 

version 1.13: Durham, N.H., University of New Hampshire, 

54 p.

Campbell, I.C., Poole, C., Giesen, W., and Valbo-Jorgensen, 

J., 2006, Species diversity and ecology of Tonlé Sap Great 

Lake: Cambodia, Aquatic Sciences, v. 68, p. 355–373.

CARIS, 2012a, CARIS HIPS and SIPSTM: Fredericton, New 

Brunswick, CARIS, accessed May 11, 2012, at http://www.

caris.com/products/hips-sips/.

CARIS, 2012b, CARIS BASE EditorTM: Fredericton, New 

Brunswick, CARIS, accessed May 11, 2012, at http://www.

caris.com/products/bathydatabase/base-editor/.

CARIS, 2011, Caris HIPS and SIPSTM 7.1 professional user’s 

guide: Fredericton, New Brunswick, CARIS, 474 p.
References Cited 21

Chaktomuk Project Management Unit and DHI Water and 

Environment, 2002, Chaktomuk area, environment, hydraulics, and morphology phase 1, appendix B—Hydrological 

and morphological report: Phnom Penh, Mekong River 

Commission, 32 p. 

Densmore, B.K., Strauch, K.R., and Dietsch, B.J., 2013, 

Hydrographic surveys of the Missouri and Yellowstone Rivers at selected bridges and through Bismarck, North Dakota, 

during the 2011 flood: U.S. Geological Survey Scientific 

Investigations Report 2013–5087, 59 p. [Also available at 

http://pubs.usgs.gov/sir/2013/5087/.]

Esri, 2012, ArcGIS 10.0: Redlands, Calif., Esri, accessed June 

2012 at http://www.esri.com/.

Falke, Martin, 2009, Flood management and mitigation 

programme component 5—Flood information based land 

management, in Proceedings of the 7th Annual Mekong 

Flood Forum, Integrated Flood Risk Management in the 

Mekong Basin, May 13–14, 2009, Bangkok, Thailand: 

Mekong River Commission Regional Flood Management 

and Mitigation Centre, p. 184–194. [Also available at http://

www.mrcmekong.org/assets/Publications/conference/7thannual-flood-forum.pdf.]

Huizinga, R.J., 2010, Bathymetric surveys at highway bridges 

crossing the Missouri River in Kansas City, Missouri, using 

a multibeam echo sounder, 2010: U.S. Geological Survey 

Scientific Investigations Report 2010–5207, 61 p. [Also 

available at http://pubs.usgs.gov/sir/2010/5207/.]

Huizinga, R.J., Elliott, C.M., and Jacobson, R.B., 2010, Bathymetric and velocimetric survey and assessment of habitat 

for pallid sturgeon on the Mississippi River in the vicinity 

of the proposed Interstate 70 Bridge at St. Louis, Missouri: 

U.S. Geological Survey Scientific Investigations Report 

2010–5017, 28 p. [Also available at http://pubs.usgs.gov/

sir/2010/5017/.]

HYPACK, Inc., 2007a, HYPACK® user’s manual 1/07: 

Middletown, Conn., HYPACK, 991 p.

HYPACK, Inc., 2007b, HYSWEEP® user’s manual 6/07: 

Middletown, Conn., HYPACK, 148 p.

Kummu, Matti, and Sarkkula, Juha, 2008, Impact of the 

Mekong River flow alteration on the Tonlé Sap flood pulse: 

Ambio, v. 37, no. 3, p. 185–192.

Mekong River Commission, 2005, Overview of the hydrology 

of the Mekong Basin: Vientiane Mekong River Commission, 73 p., accessed June 2014 at http://www.mekonginfo.

org/assets/midocs/0001968-inland-waters-overview-of-thehydrology-of-the-mekong-basin.pdf.

Mekong River Commission, 2011, Planning atlas of the Lower 

Mekong River Basin: Vientiane, Mekong River Commission, p. 1–101.

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Hydrology: Vientiane, Mekong River Commission, 

accessed April 15, 2014, at http://www.mrcmekong.org/themekong-basin/hydrology/.

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2014 at http://portal.mrcmekong.org/cms/informationservices/.

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information: Vientiane, Mekong River Commission, 

accessed June 2014 at http://ffw.mrcmekong.org/stations/

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User Service: Silver Spring, Md., National Geodetic Survey, 

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earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/

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the-world-factbook/geos/cb.html.

Glossary 23

Glossary

display depth range Setting for the RESON 

SeaBatTM 7125 that sets the maximum distance that the sonar will display. Depth range 

partially controls ping rate by estimating time 

of return for one sonar ping before a second 

sonar ping can be transmitted. The setting for 

the RESON SeaBatTM 7125 has a range from 

5 to 300 meters.

distributary A river branch flowing away 

from the main stream.

fixed Global Positioning System/Global 

Navigation Satellite System (GPS/GNSS) 

solution Position determined by the GPS/

GNSS rover with information from five or 

more GNSS satellites and with base-station 

correction information with all carrier-phase 

ambiguities resolved and the estimated position error below a set tolerance.

latency The time lapse between two different datasets. In multibeam hydrographic 

surveying, this time lapse is typically the time 

offset between sounding data and positioning 

data.

maximum ping rate Setting for the RESON 

SeaBatTM 7125 that limits the number of pings 

per second (p/s), usually used to set it slower 

than the standard ping rate for that range 

setting. The setting for the RESON SeaBatTM

7125 has a range from OFF to 50 p/s. Maximum ping rate might be increased for additional data collection over areas of interest or 

decreased to limit redundant data.

monsoonal Relating to the seasonal prevailing wind of the Indian Ocean and southern 

Asia, blowing from the southwest and bringing rain from May through September and 

blowing from the northeast from October 

through April.

outer beams The acoustic beams that are 

formed by a sonar ping on the outer edge of 

the swath. The soundings from these beams 

are revised or deleted because of the tendency 

for these data to be inconsistent with adjacent 

or overlapping data as a result of the low 

angle of incidence and high energy loss from 

absorption and spreading over the greater 

distance.

piezo-electric ceramics Ceramic material 

with piezoelectric (electric resulting from 

pressure) characteristics as well as reverse 

piezoelectric effects (internal generation of a 

mechanical strain resulting from an applied 

electric field). Reverse piezoelectric effects 

are used to produce sound waves by applying 

a voltage to the ceramic that responds with 

oscillation. The frequency of this oscillation depends on the frequency of the input 

signal and the characteristics and design of 

the ceramic material. This oscillation causes a 

series of compressions and rare-factions in the 

surrounding water—a sound pressure wave 

(RESON, Inc., 2001).

ping rate The number of sonar signals transmitted per second.

pitch Rotation of the boat about the acrosstrack axis.

pulse length Setting for the RESON SeaBatTM 7125 that controls the length of time the 

sonar signal is transmitted. Pulse length setting on the RESON SeaBatTM 7125 can range 

from 10 to 300 microseconds (μs). A narrow 

pulse length provides high resolution in shallow depths, but a wider pulse length provides 

maximum range but also low-resolution 

images.

receiver gain Setting for the RESON 

SeaBatTM 7125 that controls the increase in 

amplitude applied to the returned sonar signal 

(echo) in addition to the calculated time 

varied gain (TVG). The gain settings for the 

RESON SeaBatTM 7125 range from 0.0 to 

83.0 decibels (dB).

roll Rotation of the boat about the alongtrack axis.

swath A contiguous area of the lake, river, 

or ocean bottom that is surveyed as the boat 

makes one pass and is made up of many sonar 

pings. This area extends perpendicularly from 

the track (or traverse) and its extent is dependent on multibeam echosounder settings and 

water depth; for the RESON SeaBatTM 7125 

the angular swath width is 128 degrees, or 

approximately 4 times water depth.

transmitted power Setting for the RESON 

SeaBatTM 7125 that increases or decreases the 

amount of acoustic energy transmitted into 

the water. The RESON SeaBatTM 7125 has a 

range from OFF to 220 dB.

yaw Rotation of the boat about the vertical 

axis.
Publishing support provided by: 

Rolla Publishing Service Center

For more information concerning this publication, contact: 

Director, USGS Nebraska Water Science Center 

5231 South 19th Street 

Lincoln, NE 68512 

(402) 328–4100

Or visit the Nebraska Water Science Center Web site at: 

http://ne.water.usgs.gov

Dietsch and others—Hydrographic Survey of the Mekong, Tonlé Sap, and Bassac Rivers near Phnom Penh, Cambodia, 2012—SIR 2014–5227

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20145227